JSON_ARRAY_APPEND

CREATE TEMPORARY TABLE a (

  `data` mediumtext DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO a VALUES ('{}');

UPDATE a SET 

  data = JSON_INSERT(data, '$.messages', JSON_QUERY('[]', '$')) ,

  data = JSON_ARRAY_APPEND(data, '$.messages', JSON_QUERY('{"date":"2018-09-03","text":"mmmmmmm"}', '$'));

-- Error (4038): Syntax error in JSON text in argument 1 to function 'json_array_append' at position 15

-- BUT, If in "JSON_QUERY('[]', '$'))" array is not empty:

UPDATE a SET 

  data = JSON_INSERT(data, '$.messages', JSON_QUERY('["AAAAA"]', '$')) ,

  data = JSON_ARRAY_APPEND(data, '$.messages', JSON_QUERY('{"date":"2018-09-03","text":"mmmmmmm"}', '$'));

-- Not errors